An innerspring mattress, or other innerspring unit, generally includes a spring assembly having a plurality of spiral coil springs having a plurality of spring convolutions and arranged in a plurality of substantially parallel rows. The springs of each row are generally interconnected to their adjacent springs by lacing the springs together with cross helicals. Cross helicals are spiral coils of substantially lesser cross-sectional diameter than the spiral coil springs. The cross helicals extend transversely of the rows of the coil springs, in both the upper and lower surfaces of the unit, and lace about the end, or terminal, convolutions of adjacent coil springs.
Terminal convolutions of coil springs generally can be closed loops formed with two opposed offset portions. Coil springs formed with offset portions, such as U-shaped offset portions, on their terminal convolutions are disposed in a spring assembly so that the offset portions are adjacent one another in each row. The cross helicals then lace about adjacent offset portions.
It is desirable to provide innerspring units that conform to the shape of a supported body. Innerspring units containing coil springs which are laced together about offset portions generally have a greater conformance to a supported body than units having springs without such offset portions. The spring conformance is attributable, at least in part, to the hinging action between laced-together offset portions.
Conventional U-shaped offset portions are encircled by a cross helical about the base of the "U". The encircled bases of adjacent offset portions lie within the circumference of the cross helical, substantially parallel to the cross helical's principal axis. Although the offset portions are thus disposed in close proximity, an extensive amount of hinging action between offset portions is allowed; the bases can move past one another. This hinging action allows each coil spring to be compressed somewhat independently of its adjacent springs. The innerspring unit thus conforms to the shape of a supported body or other load.
It is also desirable to provide an innerspring unit having a great degree of firmness. Firmness can be defined as the extent of coil spring compression versus the applied load. Firmness of a unit is dependent upon many variables, such as the unit's coil count (number of coils per unit surface area) and the coil stiffness. For a given unit, raising the coil count or substituting stiffer, less resilient coil springs will decrease the extent of spring compression versus the applied load, but will generally increase the expense of manufacturing such unit.
Another method for increasing the firmness of an innerspring unit, described in U.S. Pat. No. 3,653,082, is to crimp or compress the cross helicals about the offsets of coil springs at the longitudinal side borders of the unit. The crimping frictionally engages the cross helicals to the encircled offsets, restricting the hinging action between the interconnected offsets and therefore between adjacent coil springs. This last method, however, can reduce the extent of hinging action and thus reduce spring conformation to the shape of a supported body.
It is desirable that an innerspring unit, particularly a mattress innerspring unit, be both firm and have a high degree of body conformation, without one characteristic being significantly sacrificed for the other. It is desirable to optimize the firmness and the body conformation characteristics of an innerspring unit without adding substantially to the manufacturing expense for such unit. It is also desirable to provide varying degrees of firmness and body conformation to different portions of an innerspring unit, for instance, more firmness to the longitudinal side borders and more body conformation to the center or end portions. It may also be desirable to provide additional firmness at other regions of an innerspring unit, such as the longitudinal mid-region of a very wide mattress.
It is therefore an object of the present invention to provide an improved spiral coil spring and a spring assembly containing such coil springs, which is firmer than units having conventional spring offsets and yet has the desired degree of body conformation. It is also an object of the invention to provide a spring assembly with improved coil springs in which spring conformation and firmness characteristics are optimized without substantially increasing the cost of the unit.